For instance, within the Department of Energy (DOE) complex, 335 underground tanks were employed to process and store radioactive and chemical mixed wastes generated from weapon materials production over the past 50 years. These tanks hold collectively over 90 million gallons of high-level-wastes (HLW) and low-level-wastes (LLW) in forms of sludge, saltcake, slurry, and supernate. Some of the tanks have exceeded their life expectancy and leaked to contaminate about 470 billion gallons of groundwater.
Most of these tanks contain waste with a diverse portfolio of mainly inorganic anions including nitrate, nitrite, hydroxide, sulfate, and phosphate. Temperature profiles of these tanks vary from near ambient to temperatures over 93xc2x0 C. Much of the radioactivity arises mainly from strontium, cesium, and to a lesser extent from rubidium, yttrium, barium, technetium, and actinides. Table 1 presents concentrations profiles for some of the DOE aqueous waste streams.
Cesium is the primary radioactive component found in supernatants and salts cakes. Strontium and technetium tend to be concentrated in supernatants and sludge washing liquids. Cesium and strontium are the major source of radiation and heat, while technetium tends to be very mobile in the environment and persist for a long period of time (half-life: 210,000 years). Actinides, however, tend to be concentrated in the sludge portion of the waste, and thus in the soluble portion, their concentrations are very low.
The DOE economic waste minimization strategy was centered on expending separation methods to pretreat the radioactive waste. These methods provide a sequence of processes to partition the waste into a small volume of HLW for deep geologic disposal, and a large volume of LLW for disposal in near-surface facilities. Vitrification, the process of converting materials into a glass-like substance, is currently the preferred HLW immobilization in deep geologic repository. The estimated cost of the vitrification and the repository disposal is about $1 million per canister of glass produced. As such, vitrifying wastes directly is prohibited, and the process is limited to concentrated HLW. Thus, the DOE treatment methods are focused primarily on the separation of the small quantities of radioactive species from the waste bulk. This would result in a small volume of HLW for vitrification, and a larger volume of LLW which can be disposed at a much less expensive cost.
A number of aqueous streams are encountered in tank waste treatment. These streams may include tank waste supernatant, waste retrieval sluicing water, sludge wash solutions, and the like. Several separation techniques are considered for any given tank waste stream. Solvent extraction, particularly by crown ethers, has gained significant consideration for DOE applications in separation of radioactive alkali and alkaline earth cations from liquid streams. Factors such as ion speciation in strong mixed-cation mixtures, and sensitivity to the volume and grade (solid-free) of the liquid stream are under experimentation. Traditional pressure-driven membrane processes such as nanofiltration, ultrafiltration, and microfiltration enhanced with molecular recognition agents attached to flow through membranes have been proposed and tested for selective removal of cesium, strontium, and technetium from aqueous streams. Factors such as membrane applicability and stability, separation selectivity, and capacity are critical issues that are under evaluation. Ion exchangers are currently under development for the removal of cesium and strontium from liquid waste streams. Organic and inorganic exchange materials are employed in the ion exchange process. Although both exchange materials exhibit strong retention for the cesium and strontium, factors such as chemical stability and capacity are yet to be resolved.
After retrieval of liquid waste from storage in tanks, sludge dissolution is needed. One of the specific techniques that is used by the DOE program is the enhanced sludge washing (ESW) process. The objective of the ESW is to remove key non radioactive inorganic species such as sulfate, phosphate, aluminum, and in some cases chromium from the tank waste sludge portion, leaving the radioactive species, primarily strontium and actinides, in the solid phase for vitrification. The process involves first the addition of 3 molar of sodium hydroxide to dissolve aluminum from the sludge. The sludge is then washed with an aqueous solution containing 0.01 molar sodium hydroxide and 0.01 molar sodium nitrate to remove interstitial liquid and any remaining soluble solids. The efficiency of the ESW process is questionable. First, inadequate removal of the key non-radioactive sludge components always results in production of an unacceptably large volume of HLW. Second, substantial volumes of aqueous wash solutions as well as sludge are generated which both require treatment and disposal as LLW. Inspection of Table 1 indicates high concentrations of sodium in forms of hydroxide, nitrate, and nitrite as a result of the ESW process (sodium negatively interferes with vitrification). Sludge handling and disposal is becoming more expensive as burial requirements increase and approved burial sites become less available. Third, excessive settling times of suspended solids with gel formation.
Activities such as sludge retrieval and sludge washing have resulted in excess water in tanks wastes. To conserve tank storage space and reduce the volume of the final waste form, evaporator has been demonstrated in a small scale operation to remove the generated excess water. The objective is to evaporate the aqueous waste to a level approaching the solubility limit of the dissolved salts. Conventional evaporation, however, implies high capital and operating costs. Innovative evaporation techniques are thus of a prime interest.
The DOE is currently seeking processing technologies to treat waste streams directly, or after appropriate separation, to produce environmentally stable, and regulatory acceptable final waste forms. Of particular interest are technologies that can perform highly selective separation of calcium/strontium or complexed technetium from aqueous streams (e.g., groundwater, supernate, slurry). Also, of prime importance are processes that are either capable of directly and selectively separate: (1) radioactive species such as strontium and actinides; or (2) bulk non radioactive inorganic species such as sodium and aluminum from sludge dissolution activities. Processes are also sought for the removal of radionuclides from calcined waste streams (plutonium and other actinides) at high temperatures; particularly processes that: (1) separate inorganic species into concentrated product streams; (2) can withstand a radiolytic environment; (3) can be scaled to processing at rates of 2 to 30 gallons per minute; (4) are simple to construct and operate; and (5) are economically viable.
Although some of the DOE under development emerging technologies individually remove their target contaminants effectively, these technologies would presumably be employed in series, and would each entail separate process requirements, consumption and stripping of materials, effluent streams, and different impacts on vitrification. However, in many severe cases such as the DOE waste streams, a single type system may not be the best answer. Hybrid systems to improve productivity and achieve better separation would be the optimum solutions. As such, there are compelling advantages to a single hybrid processing system that could concentrate radioactive species for small HLW, and evaporate the aqueous phase to produce an ultrapure effluent stream, leaving low volume of concentrated LLW. This invention provides an innovative hybrid process based on combining precipitation with membrane distillation concepts. The precipitation step would perform the highly selective separation of alkaline earth cations (calcium, strontium, barium, and radium), alkali earth cations (rubidium and cesium), and other radioactive species such as yttrium, technetium, and plutonium (if a xe2x80x9csuitable anionxe2x80x9d is enhanced or introduced) as a small volume of HLW. The precipitation step also has the capability to separate sodium and aluminum from the bulk stream of non radioactive species. This would enable the membrane distillation process to be efficient (minimize the radiolytic environment, and reduce the effect of osmotic pressure) in distilling ultrapure water vapor from the aqueous waste streams, and thus to leave a small volume of LLW.
A novel environmentally benign separation process which consists of coupling precipitation with membrane distillation in an integrated hybrid system is invented. The process is deemed to be efficient and cost effective with high potential for significant environmental and industrial impacts. Precipitation is the key step in which a suitable organic solvent is added to an aqueous stream containing inorganic species to form selective precipitates. This step would serve two objectives: (1) selectively separating targeted radioactive inorganics (rubidium, cesium, strontium, barium and radium,) from the bulk of inorganic aqueous stream as a small volume of HLW for direct vitrification; and (2) separating non radioactive inorganic species (sodium, aluminum and others) to reduce the osmotic pressure and the viscosity of the aqueous solution. Once the precipitation step is efficiently established, membrane distillation which is a low temperature gradient process that can take advantage of the wide variations in wastes temperatures takes place to: (1) recover the precipitation solvent (vacuum membrane distillation); and (2) produce an ultrapure effluent stream by concentrating the aqueous stream that contains dissolved inorganic species as LLW (membrane distillation).
Primary candidates for the invented process would be for the treatment of DOE waste streams. An additional application of this process beyond the scope of the DOE waste streams, would be for the treatment of produced water radioactivity Naturally Occurring Radioactive Materials: NORM) in the oil, gas, geothermal and mining industries. Other examples of potential industrial applications include the removal of sulfate and scale salts from: (1) seawater to be used as a water flood in offshore oil and gas reservoirs; (2) cooling towers blowdown streams; (3) feed and/or concentrate streams in pressure-driven membrane processes. Other examples of potential environmental applications include the removal of (1) chloride salts from contaminated groundwater with road deicing salts; (2) transition metals from landfill leachate or groundwater, and (3) other streams resulting from, for instance, plating facilities, washrack facilities, metal cleaning facilities, paint stripping facilities and laundries facilities.
In one aspect, the present invention provides a method of treating an aqueous stream having inorganic materials dissolved therein, the inventive method comprising the steps of (a) adding organic solvent to the aqueous stream in an amount effective to form a precipitate comprising at least a portion of the inorganic material; (b) removing at least most of the organic solvent from the aqueous stream; (c) removing at least most of the precipitate from the aqueous stream to produce an intermediate aqueous product; and (d) distilling the intermediate aqueous product by membrane distillation to produce an aqueous permeate product.
In another aspect, the present invention provides a method of treating an aqueous stream having inorganic material dissolved therein, the inventive method comprising the steps of (a) distilling the aqueous stream by membrane distillation to produce an aqueous permeate product and an intermediate concentrate comprising at least most of the inorganic material; (b) adding an organic solvent to the intermediate concentrate in an amount effective to form a precipitate comprising at least a portion of the inorganic material; (c) removing at least most of the organic solvent from the intermediate concentrate; and (d) removing at least most of the precipitate from the intermediate concentrate to produce a concentrate product and an at least partially purified aqueous product.
Examples of suitable organic solvents employed in the present invention include: isopropylamine, ethylamine, propylamine, diisopropylamine, diethylamine, dimethylamine, and combinations thereof. The organic solvent is preferably isopropylamine, ethylamine, or a combination thereof.
In one aspect, at least a portion of the inorganic material removed by the inventive method can include, but is not limited to, rubidium, cesium, strontium, francium, scandium, yttrium, lanthanum, actinium, chromium, cobalt, cadmium, mercury, nickel, zinc, iron, europium, cerium, praseodymium, neptunium, plutonium, americium, curium, nobelium technetium, ruthenium, iodine (I-129), carbon (C-14), tritium (H-3), cyanide, and combinations thereof.
In another aspect, the present invention provides a method of treating an aqueous stream having inorganic material dissolved therein, the inventive method comprising the steps of: (a) removing volatile organic material from the aqueous stream by vacuum membrane distillation; (b) adding organic solvent to the aqueous stream in an amount effective to form a precipitate comprising at least a portion of the inorganic material; (c) removing at least most of the organic solvent from the aqueous stream; and (d) removing at least most of the precipitate from the aqueous stream.
In another aspect, the present invention provides a method of treating an aqueous stream having inorganic material dissolved therein, the inventive method comprising the steps of: (a) removing volatile organic material from the aqueous stream by vacuum membrane distillation; (b) distilling the aqueous stream by vacuum membrane distillation to produce an aqueous distillate product and a concentrate comprising at least most of the inorganic material; and (c) adding an organic solvent to the concentrate in an amount effective to precipitate at least a portion of the inorganic material; and (d) removing at least most of the precipitate from the concentrate.
In another aspect, the present invention provides a method of producing petroleum, gas, or other products from a subterranean formation using seawater. The inventive method comprises the steps of (a) removing natural sulfate from the seawater; and (b) injecting the resulting treated seawater product into the subterranean formation. Natural sulfate is removed from the seawater in step (a) by (i) adding organic solvent to the seawater in an amount effective to form a precipitate comprising the sulfate, and (ii) removing the precipitate from seawater to produce the treated seawater product. The organic solvent employed in the inventive method is preferably isopropylamine, ethylamine, or a combination thereof.
In yet another aspect, the present invention provides a method of producing petroleum, gas, or other products from a subterranean formation using formation-produced water. The inventive method comprises the steps of: (a) removing natural, inorganic material from the formation-produced water; and (b) injecting the resulting treated water product into the subterranean formation. In step (a), the natural, inorganic material is removed from the formation-produced water by (i) adding organic solvent to the formation-produced water in an amount effective to form a precipitate comprising the inorganic material and (ii) removing the precipitate from the formation-produced water to yield the treated water product. The organic solvent employed in the inventive method is preferably isopropylamine, ethylamine, or a combination thereof The natural inorganic material contained in the formation-produced water will typically comprise at least one of barium, strontium, radium, and Naturally Occurring Radioactive Material.
Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments.